UofT-DSI | sql - Assignment 2

02_activities/assignments/Assignment2.md

@@ -54,7 +54,50 @@ The store wants to keep customer addresses. Propose two architectures for the CU
 **HINT:** search type 1 vs type 2 slowly changing dimensions. 
 
 ```
-Your answer...
+
+
+Module: SQL
+Assignment: 2
+Section: 1
+Prompt: 3
+Name: Chun-Yuan Chen
+
+
+SCD Type 1: Overwriting the old address with the new one (i.e., old records overwritten)
+
+|customer_id|province|city           |street_name     |street_number|unit_number|postal_code| last_update_date |
+|-----------|--------|---------------|----------------|-------------|-----------|-----------|------------------|
+| 566       | ON     | Toronto       | Yonge Street   | 12          | 503       |M5E 1R4    |2025-08-15        |
+| 889       | ON     | Richmond Hill | Yonge Street   | 8868        | 702E      |L4C 1Z8    |2025-08-15        |
+
+In a Type 1 architecture, when a customer's address changes, the old address is overwritten with the new one, 
+in the general case so the table keeps only the most recent address for each customer. 
+In the illustrated example above, I put an 'last_update_date' column so can see when the address was last changed.
+
+
+SCD Type 2: While Keeping the old address, creating new rows for the new one (i.e., changes retained)
+
+|customer_id|province|city           |street_name     |street_number|unit_number|postal_code|effective_date_start|effective_date_end|
+|-----------|--------|---------------|----------------|-------------|-----------|-----------|--------------------|------------------|
+| 566       | ON     | Markham       | Main Street N  | 68          | 311       |L3P 0N5    |2023-01-25          |2025-08-14        |
+| 566       | ON     | Toronto       | Yonge Street   | 12          | 503       |M5E 1R4    |2025-08-15          |NULL              |
+| 889       | ON     | Hamilton      | Barton Street E| 2782        | 814       |L8E 2J8    |2020-06-17          |2025-08-14        |
+| 889       | ON     | Richmond Hill | Yonge Street   | 8868        | 702E      |L4C 1Z8    |2025-08-15          |NULL              |
+
+In contrast to the Type 1 architecture, the Type 2 architecture retains all the old addresses in the table when a customer's address changes and 
+adds the new one as a new row. In addition, two date columns (i.e., 'effective_date_start' and 'effective_date_end') 
+present the effective period for each address. For the current address, the 'effective_date_end' column is NULL because it is still active.  
+
+
+In my personal view, if the bookstore is small and has very limited storage resources, the Type 1 architecture would be easier to manage and query. 
+However, the Type 2 architecture offers a window to review past records, 
+which can be useful for checking back logistics and delivery issues that occurred before the address update.
+
+
+Chun-Yuan Chen
+2025-08-15
+
+
 ```
 
 ***

02_activities/assignments/assignment2.sql

@@ -1,6 +1,16 @@
+
+--Module: SQL
+--Name: Chun-Yuan Chen
+--Assignment: 2
+--Sections: 2 & 3
+
+
+
 /* ASSIGNMENT 2 */
 /* SECTION 2 */
 
+
+
 -- COALESCE
 /* 1. Our favourite manager wants a detailed long list of products, but is afraid of tables! 
 We tell them, no problem! We can produce a list with all of the appropriate details. 
@@ -12,14 +22,33 @@ product_name || ', ' || product_size|| ' (' || product_qty_type || ')'
 FROM product
 
 But wait! The product table has some bad data (a few NULL values). 
-Find the NULLs and then using COALESCE, replace the NULL with a 
-blank for the first problem, and 'unit' for the second problem. 
+Find the NULLs and then using COALESCE, replace the NULL with a blank for the first column with nulls, and 
+'unit' for the second column with nulls.
 
 HINT: keep the syntax the same, but edited the correct components with the string. 
 The `||` values concatenate the columns into strings. 
 Edit the appropriate columns -- you're making two edits -- and the NULL rows will be fixed. 
 All the other rows will remain the same.) */
 
+SELECT
+	/* Notes: 
+	1. Initially, I checked for any empty strings in each separate column of interest, 
+	   and if found, converted them to NULLs. 
+	2. In this case, I see these lines of code as a data cleaning step, although sometimes a blank can 
+	   represent specific meaning depending on the context. 
+	*/ 
+	NULLIF(product_name, '') AS product_name,          
+	NULLIF(product_size, '') AS product_size,                
+	NULLIF(product_qty_type, '') AS product_qty_type, 
+
+COALESCE(product_name, '') || ', ' || COALESCE(product_size, '') || ' (' || COALESCE(product_qty_type, 'unit') || ')' AS product_list
+	/* Notes: 
+	1. Although the product_name column contains no NULLs, I still applied COALESCE for consistency and to ensure robustness.
+	2. In the new product_list column, the two NULLs originally in product_size have now been replaced with blank.
+	3. In the new product_list column, the two NULLs originally in product_qty_type have now been replaced with 'unit'.
+	*/
+FROM product;
+
 
 
 --Windowed Functions
@@ -32,17 +61,36 @@ each new market date for each customer, or select only the unique market dates p
 (without purchase details) and number those visits. 
 HINT: One of these approaches uses ROW_NUMBER() and one uses DENSE_RANK(). */
 
+SELECT DISTINCT customer_id, market_date, /* Notes: I added DISTINCT to ensure that only unique market dates per customer are returned. */
+DENSE_RANK() OVER (PARTITION BY customer_id ORDER BY market_date ASC) AS visit_number_asc
+	/* Notes: Based on the question, I assumed that multiple transactions on the same date, regardless of time, 
+	   count as the same visit. Therefore, I did not bring transaction_time into the code.*/
+FROM customer_purchases;
+
 
 
 /* 2. Reverse the numbering of the query from a part so each customer’s most recent visit is labeled 1, 
 then write another query that uses this one as a subquery (or temp table) and filters the results to 
 only the customer’s most recent visit. */
 
+SELECT customer_id, market_date
+FROM (
+	SELECT DISTINCT customer_id, market_date,
+	DENSE_RANK() OVER (PARTITION BY customer_id ORDER BY market_date DESC) AS visit_number_desc 
+	/* Notes: I used DESC to ensure each customer’s most recent visit is labeled 1. */
+        FROM customer_purchases
+	)
+WHERE visit_number_desc = 1; /* Notes: Now, only the most recent visit for each customer is returned. */
+
 
 
 /* 3. Using a COUNT() window function, include a value along with each row of the 
 customer_purchases table that indicates how many different times that customer has purchased that product_id. */
 
+PRAGMA table_info(customer_purchases); /* Notes: I used this just to get a quick look myself at all the columns. */
+SELECT *, COUNT(product_id) OVER (PARTITION BY customer_id, product_id) AS product_purchase_count
+FROM customer_purchases;
+
 
 
 -- String manipulations
@@ -51,16 +99,26 @@ These are separated from the product name with a hyphen.
 Create a column using SUBSTR (and a couple of other commands) that captures these, but is otherwise NULL. 
 Remove any trailing or leading whitespaces. Don't just use a case statement for each product! 
 
-| product_name               | description |
-|----------------------------|-------------|
-| Habanero Peppers - Organic | Organic     |
+| product_name                          | description |
+|---------------------------- |-------------|
+| Habanero Peppers - Organic  | Organic     |
 
 Hint: you might need to use INSTR(product_name,'-') to find the hyphens. INSTR will help split the column. */
 
+SELECT product_name,
+	CASE 
+		WHEN INSTR(product_name, '-') THEN TRIM(SUBSTR(product_name, INSTR(product_name, '-')+1))
+		ELSE NULL
+	END AS description
+FROM product;
+
 
 
 /* 2. Filter the query to show any product_size value that contain a number with REGEXP. */
 
+SELECT * FROM product
+WHERE product_size REGEXP '[0-9]';
+
 
 
 -- UNION
@@ -73,6 +131,30 @@ HINT: There are a possibly a few ways to do this query, but if you're struggling
 3) Query the second temp table twice, once for the best day, once for the worst day, 
 with a UNION binding them. */
 
+DROP TABLE IF EXISTS temp.sales_values_by_date; /* Notes: I found temp. appears to be not necessarily required. */ 
+CREATE TEMP TABLE sales_values_by_date AS
+SELECT market_date, SUM(quantity * cost_to_customer_per_qty) AS total_sales_values
+FROM customer_purchases
+GROUP BY market_date;
+
+
+DROP TABLE IF EXISTS temp.sales_values_ranked; 
+CREATE TEMP TABLE sales_values_ranked AS
+SELECT market_date, total_sales_values,
+       RANK() OVER (ORDER BY total_sales_values DESC) AS total_sales_values_desc,
+       RANK() OVER (ORDER BY total_sales_values ASC) AS total_sales_values_asc
+FROM sales_values_by_date;
+
+
+SELECT market_date, total_sales_values, 'best day' AS total_sales_values_marked
+FROM sales_values_ranked
+WHERE total_sales_values_desc = 1
+
+UNION
+
+SELECT market_date, total_sales_values, 'worst day' AS total_sales_values_marked
+FROM sales_values_ranked
+WHERE total_sales_values_asc = 1;
 
 
 
@@ -90,27 +172,97 @@ How many customers are there (y).
 Before your final group by you should have the product of those two queries (x*y).  */
 
 
-
+/* Notes:
+	1. This question is really no walk in the park, pretty hard!
+	2. Original tables needed: customer, vendor, product, vendor_inventory
+	3. Derived tables in my case: all_possible_vendor_product_pairs, vendor_original_prices, how_much_vendor_make_per_product.
+*/
+
+WITH 
+total_number_customers AS (
+	SELECT COUNT(DISTINCT c.customer_id) AS num_customers FROM customer c), 
+	/* Notes: Get total #customers first, 26, and apply this number later, 
+	   because the question highlighted 'every customer on record'.  */
+
+all_possible_vendor_product_pairs AS (
+	SELECT v.vendor_id, v.vendor_name, p.product_id, p.product_name FROM vendor v
+	CROSS JOIN product p), 
+	/* Notes: Get all possible vendor-product pair, 9 vendors x 23 products, 207 pairs. */
+
+vendor_original_prices AS (
+	SELECT DISTINCT vendor_id, product_id, original_price FROM vendor_inventory), 
+	/* Notes: Get each original price for each of the products listed from the three vendors in this table. */
+
+how_much_vendor_make_per_product AS (
+	SELECT 
+		apvpp.vendor_id, 
+		apvpp.vendor_name, 
+		apvpp.product_id, 
+		apvpp.product_name, 
+		vop.original_price, 
+		tnc.num_customers,
+		5 * vop.original_price * tnc.num_customers AS vendor_revenue
+
+	FROM all_possible_vendor_product_pairs apvpp
+		LEFT JOIN vendor_original_prices vop ON apvpp.vendor_id = vop.vendor_id AND apvpp.product_id = vop.product_id
+		CROSS JOIN total_number_customers tnc)
+	/* Notes: Derive the revenue variable. */
+
+SELECT vendor_name, product_name, original_price, num_customers, COALESCE(vendor_revenue, 0) AS vendor_revenue
+FROM how_much_vendor_make_per_product
+WHERE original_price IS NOT NULL
+ORDER BY vendor_name, product_name;
+
+
+
 -- INSERT
 /*1.  Create a new table "product_units". 
 This table will contain only products where the `product_qty_type = 'unit'`. 
 It should use all of the columns from the product table, as well as a new column for the `CURRENT_TIMESTAMP`.  
 Name the timestamp column `snapshot_timestamp`. */
 
+DROP TABLE IF EXISTS product_units;
+CREATE TABLE product_units AS
+SELECT *, DATETIME('now', 'localtime') AS snapshot_timestamp
+FROM product
+WHERE product_qty_type = 'unit';
+
+SELECT * FROM product_units; /* Notes: This line of code just for myself to do a check. */
+
 
 
 /*2. Using `INSERT`, add a new row to the product_units table (with an updated timestamp). 
 This can be any product you desire (e.g. add another record for Apple Pie). */
 
+INSERT INTO product_units (product_id, product_name, product_size, product_category_id, product_qty_type, snapshot_timestamp)
+VALUES (3, 'Poblano Peppers - Organic', 'large', 1, 'unit', DATETIME('now', 'localtime'));
+	/* Notes: So, now there are two same records (product_id = 3) except snapshot_timestamp, 
+	          one is old and the other new in my case. */
+
+SELECT * FROM product_units; /* Notes: This line of code just for myself to do a check. */
+
 
 
 -- DELETE
 /* 1. Delete the older record for the whatever product you added. 
 
 HINT: If you don't specify a WHERE clause, you are going to have a bad time.*/
-
-
-
+
+DELETE FROM product_units AS pu1
+WHERE pu1.snapshot_timestamp < (
+	SELECT MAX(snapshot_timestamp)
+	FROM product_units AS pu2
+	WHERE pu2.product_id = pu1.product_id
+	AND pu2.product_name = pu1.product_name
+	AND pu2.product_size = pu1.product_size
+	AND pu2.product_category_id = pu1.product_category_id
+	AND pu2.product_qty_type = pu1.product_qty_type
+);
+
+SELECT * FROM product_units; /* Notes: This line of code just for myself to do a check. */
+
+
+
 -- UPDATE
 /* 1.We want to add the current_quantity to the product_units table. 
 First, add a new column, current_quantity to the table using the following syntax.
@@ -128,6 +280,39 @@ Finally, make sure you have a WHERE statement to update the right row,
 	you'll need to use product_units.product_id to refer to the correct row within the product_units table. 
 When you have all of these components, you can run the update statement. */
 
+ALTER TABLE product_units ADD current_quantity INT; 
+SELECT * FROM product_units; /* Notes: This line of code just for myself to do a check. */
+
+DROP TABLE IF EXISTS vendor_inventory_copy; 
+CREATE TABLE vendor_inventory_copy AS SELECT * FROM vendor_inventory; 
+/*Notes: I made a copy to vendor_inventory, didn't want to affect the original one. */
+
+ALTER TABLE vendor_inventory_copy ADD COLUMN current_quantity INT;
+SELECT * FROM vendor_inventory_copy; /* Notes: This line of code just for myself to do a check. */
+
+UPDATE vendor_inventory_copy 
+/* Notes: It appears to not able to use alias for vendor_inventory_copy in update command here. */
+SET current_quantity = (
+    SELECT quantity
+    FROM vendor_inventory vi
+    WHERE vi.product_id = vendor_inventory_copy.product_id
+    ORDER BY market_date DESC
+    LIMIT 1
+);
+
+SELECT * FROM vendor_inventory_copy; /* Notes: This line of code just for myself to do a check. */
+
+
+DROP TABLE IF EXISTS vendor_inventory_current_quantity;
+CREATE TABLE vendor_inventory_current_quantity AS 
+SELECT DISTINCT product_id, current_quantity 
+FROM vendor_inventory_copy;
 
 
+UPDATE product_units
+SET current_quantity = COALESCE(
+	(SELECT vicq.current_quantity FROM vendor_inventory_current_quantity vicq WHERE vicq.product_id = product_units.product_id), 
+     0); /* Notes: If not matched, then just use 0 instead. */
+
+SELECT * FROM product_units; /* Notes: This line of code just for myself to do a check. */